1. Field of the Invention
The present invention relates to micromechanical rate-of-rotation sensors based upon the Coriolis principle. More particularly, the invention pertains to a sensor including two electrostatically stimulated plate-like oscillators that oscillate out-of-phase.
2. Description of the Prior Art
The measurement of rotation rates by Coriolis forces is known and used in numerous sensors. The coriolis rate-of-rotation sensors currently available on the market make use of piezoelectric effects, for example, for driving oscillations and for readout. See, for example, EP 0 563 761 A1, EP 0 563 762 A1, EP 0 520 467 A1, EP 0 520 468 A2, EP 0 533 163 A2, EP 0 460 089 B1, GB 2 251 072 A, CA 1 313 065 A, EP 0 298 511 B1, EP 0 318 972 B1, EP 0 638 783 A1 and U.S. Pat. No. 5,247,252.
As the piezoelectric materials employed in microengineering for the above purposes have temperature-dependent material parameters which differ significantly from silicon, high temperature dependencies and non-reproducibilities of the zero point of such rotation rate sensors result that severely restrict their fields of application.
Rotation rate sensors with micromechanical elements are available. On the other hand, purely micromechanical solutions for a compact sensor are not as yet on the market. However, micromechanical solutions in which coriolis accelerations are measured are known from patent publications see, for example, U.S. Pat. No. 5,101,702, CH 682 844 A5, GB 2 251 688 A, DE 40 22 495 A1, EP 0 574 143 A1, EP 0 634 629 A1, U.S. Pat. No. 5,203,208, EP 0 442 280 A2, U.S. Pat. No. 4,750,364, EP 0 623 807 A, EP 0 620 415 A1, GB 2 276 241 A, U.S. Pat. No. 4,884,446 and DE 40 41 582 A1.
Capacitive actuators are suggested for driving capacitive bridge circuits for readout of oscillating structures in a micromechanical design of silicon technology. Note, for example, GB 92 009 30, EP 0 586 437 A1, U.S. Pat. No. 5,207,685, DE 40 41 582 A1.
The most effective force direction for the capacitive drive of an oscillating micromechanical structure is the vertical one between two opposed, oppositely polarized plates. For this reason it is expedient, for such an oscillating structure, to select an arrangement in which (referring to FIG. 4 of this application) two oppositely-polarized plates 100 and 200, respectively, can be employed to drive the oscillator 101. The schematic signal diagrams beneath the cross-sectional illustration of the oscillator clarify the phases of the excitation signals applied to the upper 100 (electrode 1) and the lower plate 200 (electrode 2) respectively.
If very small distances should exist between the two drive capacitor surfaces between the upper plate 100 and the oscillator 101 and between the lower plate 200 and the oscillator 101, then sufficiently large driving forces may be obtained with comparatively small voltages (e.g. 5 V). The disadvantage of such arrangement, known in principle, according to FIG. 4 (cf., for example, U.S. Pat. No. 4,884,446), is that the small distances between the drive capacitor surfaces simultaneously restrict the maximum oscillation amplitude of the oscillator 101 to a fraction of the capacitor plate spacings. For small oscillator attenuations, very strict requirements must be imposed upon the gas atmosphere within which the oscillator arrangement is located.
Under the influence of coriolis acceleration, a deflection orthogonal to the plane of oscillation is generated and can be used to measure rotation rate. In the example of FIG. 4, the indicated rotation rate generates coriolis forces that point out of the plane of the figure. However, this arrangement has the disadvantage that the reaction forces to the oscillator movement are introduced into the environment of the sensor, and can bring about interference with the measured values.
Double oscillator arrangements as shown by FIG. 5 (cf. also GB 2 251 688 A) have been proposed to avoid this. The first oscillator 50 and the second oscillator 60 move out-of-phase. A rate of rotation whose vector is perpendicular to the plane of the frame produces torques about this common axis of rotation 55 at the oscillation frequency. Employing this known arrangement, it is possible to achieve large oscillator amplitudes. However, this arrangement has the disadvantage that, given capacitive drive via differential capacitors, the driving forces act between surfaces located perpendicular to the plane of the wafer and with small separations exactly parallel to one another produced only with difficulty. Non-uniform air gaps at the oscillators 50, 60 provide a stimulus about the axis of rotation and influence the zero point of the sensor considerably. This effect is temperature-dependent. A further disadvantage of this arrangement with capacitive stimulation consists of the relatively small driving forces. Attempts are being made, according to more recent proposals, to reduce these by means of comb-like electrodes.